The invention relates to coherent optical sources and notably to solid state lasers.
These lasers have a cavity in which it is possible to insert solid materials such as glasses or crystals doped with rare earth ions which, when photoexcited, generate radiating emissions whose efficiency varies with the frequency of emission. Certain applications relate to well-determined frequencies and, in this case, they can be hampered by other, more efficient radiating emissions. This is why, when a source emits at least two radiating emissions at frequencies f.sub.1 and f.sub.2, and when the frequency f.sub.2 is located in the spectral band with which the application is concerned, the cavity should include a filtering device so as to generate only the desired emission at the frequency f.sub.2. This type of problem arises notably in sectors such as those of target designation or telemetry or, again, industrial forming which use lasers that should meet the criteria of eye safety. Indeed, the optical damage threshold of the eye in terms of energy is higher in the 1.4-1.6 .mu.m window. To this end, it is necessary to have available coherent optical sources that emit in this spectral window. However, if sources of this type emit at other frequencies, it becomes indispensable to eliminate these other frequencies. Among the sources that emit in the 1.4.1.6 .mu.m spectral window, there are coherent optical sources comprising a material such as the YAG crystal doped with Nd.sup.3+ ions which has the valuable feature of being a system with more than three levels of energy. These are levels between which the radiating emissions at several frequencies are furthered by the presence of a non-radiating emission with lower energy that prevents saturation between the excited states. However, the emission wavelengths of an Nd.sup.3+ doped YAG crystal (Y.sub.3 Al.sub.5 O.sub.12) are numerous: EQU L.sub.1 =0.946 .mu.m, L.sub.2 =1.06 .mu.m, L.sub.3 =1.318 .mu.m, L.sub.4 =1.8 .mu.m
and more specifically, each energy level corresponding to these wavelengths is actually constituted by a set of sub-levels due to the effect of the crystal field on the electronic states of the Nd.sup.3+ ion (Stark effect). Thus, there are several wavelengths of radiating emission in the neighborhood of L.sub.3 which are more specifically the wavelengths L.sub.3i indicated in micrometers in the table 1 here below, with their respective efficiency observed during lamp pumping on the different emission lines. The efficiency is indicated in terms of relative value, and more specifically in terms of percentage of energy totally emitted on all the levels L.sub.3i.
TABLE 1 __________________________________________________________________________ 1.3188 1.3200 1.3338 1.3350 1.3382 1.3410 1.3564 1.4140 1.4440 L.sub.3i i = 1 i = 2 i = 3 i = 4 i = 5 i = 6 i = 7 i = 8 i = 9 __________________________________________________________________________ Effic. 34 9 13 15 24 9 14 1 0.2 __________________________________________________________________________